Surviving at the highest and coldest: Nutritional and chemical components of fallback foods for Yunnan snub‐nosed monkeys

Abstract Fallback foods (FBF), categorized into staple and filler types, are suboptimal food sources chosen by animals in response to a scarcity of preferred food items during specific periods. Using lichens as FBF by Yunnan snub‐nosed monkeys (Rhinopithecus bieti) represents a distinctive ecological adaptation and evolutionary development within nonhuman primates. This study delves into the annual dietary choices of the species to address issues, elucidate the nutritional value, and understand the ecological significance of lichens for this primate species, which resides at the highest altitudes and experiences the coldest weather among global primates. The findings reveal that the lichens consumed by the monkeys serve as the staple FBF, with Bryoria spp. and Usnea longissima being the primary dietary species. The former is the preferred choice, providing higher digestible fiber (neutral detergent fiber) levels but lower tannin, fat, ADF, and energy levels. During the dry season, lichens dominate as the monkeys' primary food and nutritional resources. In the wet season, they act as a fundamental food selection rather than an ideal dietary choice, substituting nutrients from fruits, seeds, and leaves. Compared to other Asian colobine counterparts, this species exhibits the highest lichen consumption but the lowest proportions of leaves, flowers, and seeds. This study provides valuable evidence and information for developing or amending conservation strategies and guidelines for the dietary management of captive breeding of monkeys, one of the world's critically endangered primate species.


| INTRODUC TI ON
Animals' dietary selection critically depends on external modifications in the environment, ecology, evolution, climate, habitat, air pollution, and internal population conspecific modifications in the dynamics of behaviors and interactions between populations or groups, which fluctuate periodically and seasonally.They, together, have affected animals' survivability and expect their development in the foreseeable future (Cao, 1989;Heiduck, 1997;Wang et al., 2020).Thus, understanding such external and internal changes is crucial for comprehending population ecology, evolutionary processes, and the overall functioning of ecosystems for a specific species.Thus, it is necessary to study alternative selection mechanisms, such as those revealing strategies to adopt different food resources and ecology (Litvaitis, 2000), and explore factors influencing such selection (Chapman, 1990).Such exertion can also help us identify the intensity of inter-and intra-specific competition for food and analyze population division, fusion, and fluctuation affected by natural resources, environment, ecology, and habitats under the principles and regulations of natural selection and environment adaptation (Kirkpatrick, 1996;Van Schaik & Van Noordwijk, 1988).Such effort also enhances the understanding of other ecological aspects of animals for maintaining tangible conservation and management strategies (Liu et al., 2013).
Fallback food (FBF) is a low-quality food resource chosen by animals due to the shortage of preferred food in a specific period (Marshall et al., 2009;Marshall & Wrangham, 2007).It has been considered to have two types: "staple" and "filler."The former is consumed year around and seasonally can constitute up to 100% of the diet, and the latter never constitutes 100% (Marshall & Wrangham, 2007).Lichen consumed by R. bieti has been regarded as the FBF (Grueter, Li, Ren, Wei, Xiang, & van Schaik, 2009;Huang et al., 2017), a specific habitat-dependent evolutionary selection.
Current research on the FBF of primates focuses mainly on the definition, function, ecologic, and evolutionary adaptation of different species (Constantino & Wright, 2009;Grueter, Li, Ren, Wei, Xiang, & van Schaik, 2009;Laden & Wrangham, 2005;Lambert et al., 2004;Marshall et al., 2009;Marshall & Leighton, 2006;Marshall & Wrangham, 2007).Nevertheless, attention to nutritional and chemical components that differ among FBF species and present periodic variation seems to have not been seen.Their identification and amounts consumed from specific foods are gravely required to understand animals' specific demands to maintain a normal metabolism (Mattson Jr., 1980;Schoener, 1971), especially regarding protein, fat, fibers from which carbohydrate comes, and vitamins (Hale et al., 2018;Raubenheimer & Simpson, 2016).On the other hand, the exact demands for each item recorded from a given wild animal taxon provide essential guidelines for its captivity feeding (Hansell et al., 2020).
Among primates, colobines (Colobinae) food choices have been considered to primarily consist of leaves, fruits, flowers, and seeds (Huang et al., 2010;Kirkpatrick, 1996;Oates et al., 1980).The evolutionary development of Asian colobine ancestors from Africa to Asia during the Miocene and Plioce has dramatically shaped their morphology, ecology, and dietary selection (Zhang et al., 2022).

Their phylogenetic development and distribution expansion in East
and Southeast Asia have allowed them to adapt to alternative environments, ecology, and habitats and display a significant variation of altitudinal range from sea coastlines to more than 4000 m in Mt.
Hengduan and the Qinghai-Tibet Plateau, where R. bieti resides (Peng et al., 1993).Thus, this species is unique in colobines and the world's primates due to its specific adaptation to the harsh ecology surrounded by low temperature and hypoxia, besides scarce food resources (Grueter, Li, Ren, Wei, Xiang, & van Schaik, 2009).It is one of the critically endangered primate species on the Red List of IUCN (Long, Bleisch, & Richardson, 2020).
The frigid climate has made R. bieti require tremendous energy to maintain body temperature (Guo et al., 2018;Hou et al., 2020), especially during energy-hungry periods in which food resources are scarce and variable due to seasonal climate changes, especially prolonged winter (Huang et al., 2012;Kirkpatrick, 1996;Van Schaik & Brockman, 2005).Thus, understanding how this primate species copes with such an environment and ecology through dietary selection and recording its nutritional and chemical components will provide valuable information and evidence for their conservation and captivity feeding before releasing them into the wild.
Therefore, the primary purposes of this study include: selection compared with other Asian colobines; (3) understand the selective strategy of the lichen groups in terms of the nutritional and chemical components; and (4) provide scientific information in amending or making conservation strategies and guidelines for dietary management for the creatures in captivity feeding programs.

| FIELDWORK AND MATERIAL
The study sites included five species groups in Mt.Lasha, from the south to the north of the species distribution, with an altitudinal variation between 2450 and 3600 m.The vegetation primarily consists of deciduous broad-leaved forest, coniferous broad-leaved mixed forest, and alpine dark coniferous forest from lower to higher altitudes.The distance between the groups is from 10 to 20 km.Each group's average home range, the daily activity range, is between 750 and 15,100 m.A typical group of the species consists of several onemale and multi-female breeding units (OMU) and one or two all-male (bachelor) units (AMU).
The feeding habits of the monkeys were observed and recorded using a Leica 77 telescope (Solms, Germany) with instantaneous scanning sampling at 10-min intervals.From May 2008 to April 2009 and September 2015 to August 2016, we tracked the monkeys of different groups in the morning, from when they left the sleeping site to when they entered the sleeping place in the afternoon.We followed each individual for at least 5 s to record which dietary items-fruits, seeds, insects, flows, and which lichen species-were consumed by the monkeys: (1) lichens (Bryoria spp.and U. longissima), which are significantly morphologically distinguishable (Figure 1).Bryoria spp.mainly covers tree trunks, with black colors, while U. longissima is principally intertwined among tree branches with suspending thread-like white fibers; (2) dicotyledonous leaves and buds; (3) mature leaves (including bamboo leaves); (4) flowers; (5) fruits and seeds; (6) insects; (7) others, with discriminating methods described by Huang et al. (2017), at least 5 days or 50 h of eating data were recorded every month for each of the groups.

| Laboratory work
From July to November 2015, between 11 and 16 samples of Bryoria spp.and U. longissima were collected randomly at different altitudes (2450-3450 m) of the five study sites.Each sample was classified and analyzed for its nutrient and chemical contents, including crude protein (CP), fat, all fiber fractions [neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL)], ash, water-soluble carbohydrate (WSC), and starch (Hou et al., 2018).
We first determined the physical differentiation between the two lichen species consumed, then gauged each nutritional (CP, fat, all fiber fractions, ash, WSC, and starch) or chemical (tannin) component (percentage) contained in each species.All components were measured with dry lichens.The chemical component, tannin, was determined in the Folin-Ciocalteau phenol reagent (Box, 1983).The average of all samples was used in the calculation.
The food availability index (FAI), which measures the relationship between food supply and population structure (Hutto, 1990;Ritzema et al., 2017), was used to assess the relative abundance of two lichen groups.We calculated this index from June 2008 to September 2009 (Huang et al., 2012).

| Data analysis
The following procedures were used to determine the amount of each component consumed by the monkeys from FBF, either from U. longissima or Bryoria spp.: total nonstructural carbohydrates (TNC) and energy were calculated (Hou et al., 2018).The energy value of NDF was estimated using a previously published NDF digestibility coefficient of 91% (Kirkpatrick et al., 2001).
A ratio was created for each lichen species in FBF: where a is the physical proportion of U. longissima or Bryoria spp. in FBF, recorded from the wild; Ua is the physical proportion of U. longissima; and Ba is the same proportion of Bryoria spp.
The actual amount of a given component consumed from each of the lichen species from FBF was calculated with the following formula: where U% is the component percentage contained in U. longissima, and B% is the component percentage included in Bryoria spp.They were assessed in the laboratory.Thus, the total amount regarding a specific component from both lichen groups is: For example, if FBF consists of 67% of U. longissima, and 25% of Bryoria spp., recorded from the field, R (U. longissima) = Ua/(Ua + Ba) = 67/ (67 + 25) = 0.73; R (Bryoria spp.) = Ub/(Ua + Ba) = 25/(67 + 25) = 0.27.If component tannin in U. longissima is 4.5% and 6.5% in Bryoria spp., separately, from the laboratory records, the actual amount of tannin in FBF from U. longissima = 0.045 × 0.73 = 3.29%; and that from Bryoria spp.= 0.065 × 0.27 = 1.76%.
Separately, R. bieti consumes 3.29% and 1.76% of tannin from U. longissima and Bryoria spp., with a total of 5.05% from the lichens.
We assessed the relative abundance of two lichen groups (FAI) with the Mann-Whitney U-test to compare the time difference of feeding on Bryoria spp. to that on U. longissima and analyze the differences in components.Data were processed by SPSS 21.0, and the significance level was set to p < .05.All the results are shown as mean ± SD.

| RE SULTS
The first result of the study is the dietary components of R. bieti, which are summarized in the five groups.This monkey species feeds 78.9% of lichens, 4.0% of buds and young leaves, 5.1% of mature leaves, 7.3% of fruits, 1.5% of flowers, 1.9% of insects, and 1.3% of others.As for the lichen amounts, there is a significant difference between Bryoria spp.and U. longissima (U < .001,n1 = n2 = 24, p < .001)-ahigher proportion of Bryoria spp.(64.63% ± 13.28%, and 69.86% ± 10.93%) than U. longissima (14.48% ± 12.12%, and 17.11% ± 11.56%).A year-over-year variation of the lichen consumption and a comparison between the two species collected from the five groups are provided in Table 1.The results indicate that the year variation of lichens consumed by the monkeys did not reach a significant level for each lichen species.
Concerning the FAI, both lichen species show a stable supply all year round, except for a lower Bryoria spp. in January.Leaves present a remarkable seasonal variation and extreme scarcity from December to April (dry season, Figure 2).
When the two lichen species are combined, the difference between dry and wet seasons is illustrated in Figure 3.The dry season shows a significantly higher amount than the wet season.When they are analyzed separately, Bryoria spp.presents a significantly higher physical amount in the dry season than the wet one, but not U. longissima.

| Lichen consumption
Monthly lichen consumption is compared between five R. bieti groups from the southernmost to northernmost of the monkey's distribution range (Table 2).

| Nutritional and chemical components of the lichens
Bryoria spp.comprises significantly higher percentages of ash than U. longissima; the latter, however, includes significantly higher portions of ADF, fat, tannin, and energy than the former.After being adjusted, except for fat, monkeys consume a significantly higher amount of Bryoria spp.than from U. longissima (AU, Table 3).

| Nutritional and chemical components between the dry and wet seasons
Lichens provide more significant components in the dry season than in the wet one (Figure 4).A general decreasing trend of nutritional and mechanical components was found: TNC > NDF > starch > CP > ADF > WSC > ash > fat > ADL > tannin.

| DISCUSS ION
This study exposes the uniqueness of the dietary selection of R.
bieti, which considerably depends on lichens as a staple FBF and for their nutritional and chemical components.As expected, it provides valuable evidence and information to understand R. bieti's unique ecological selection, environmental adaptation, and evolutionary development, shaped by the most frigid habitats of nonhuman primates at the highest altitude.

| Unique dietary selection and evolutionary development
According to its evolutionary development, the unique dietary selection of R. bieti seemed to develop during the Quaternary since they settled down in Southern East Asia during the Pliocene after a long journey from Africa to East Asia via Europe during the Miocene-Pliocene (Roos et al., 2011).Its dietary choice comparison with the other Asian colobines based on a broader literature review is provided in Figure 5.  (Kirkpatrick, 1996).Such scenarios imply that lichens might be an exceptional dietary choice for the ancestors of the snob-nosed monkeys before they dispersed to the places where they dwell (Roos et al., 2011) (Figures 2, 3, and 5).

Rhinopithecus bieti and other Asian colobines originated from
Africa in the Middle Miocene and migrated into Eurasia through the gateway of North Africa, possibly in the Late Miocene, about 10 mya ago (Davies & Oates, 1994;Roos et al., 2011).They arrived at a Convergency-Divergency Center where R. bieti is located (Zhang et al., 2022).The oldest colobine fossil species associated with European and Asian taxa, Mesopithecus, unearthed in the Late Miocene (Davies & Oates, 1994;Heintz et al., 1981), was discovered recently in Zhaotong, Yunnan, in the deposit of the Late Miocene or Early Pliocene.It has been reckoned to be closely related to the taxa in Rhinopithecus, possibly R. bieti (Ji et al., 2020).
In other words, R. bieti is the only Asian colobine species located in the Qinghai-Tibet Plateaus and Mt.Hengduan, which have experienced significant tectonic changes following the accelerated uplift of the plateau and mountains (Sun et al., 2011).Other colobines, however, dispersed and radiated to other parts and are widely distributed in East Asia and South East Asia, reaching coastlines (Peng et al., 1993;Zhang et al., 2022) F I G U R E 2 FAI index of the two lichen species, leaf variation and the rainfall records during the study period.
PAN et al.
in other parts of the world, can be regarded as a species facing severe challenges year-round, divided into dry and wet seasons.It keeps the original dietary selection of the Asian snub-nosed colobines-lichens, especially from Bryoria spp.(Figure 3).This also implies that, following a decrease in temperature in the dry season, R. bieti consumes more food and energy to maintain body temperature (Bissell, 2014).Such mechanisms are the adaptation of going through the period with the scarcest resources in the high altitudinal region.
Another interesting comparison is between R. bieti and other taxa in the same genus: (1) not all species of Rhinopithecus have FBF; and ( 2) not all Rhinopithecus species choose lichens as FBF.Among five taxa, lichen FBF has been reported for R. bieti (Grueter, Li, Ren, The monthly contributing proportion of two lichen species in different seasons.
TA B L E 2 Seasonal and regional lichen variations in the diet of R. bieti.a From J to D: January, February, March, April, May, June, July, August, September, October, November, and December.
different from those adopted by R. bieti, R. roxellana, and the populations of R. trykeri on Mt.Gaoligong in China (Yang et al., 2016).

| Staple FBF of R. bieti
What is indicated by Table 2 shows that lichens are the most dominant food type of the monkeys analyzed (81.81 ± 8.32% of annual dietary consumption), and there is a stable supply among the five groups.Such a portion in the dry season can reach 100%.That is to say, lichens are the monkeys' dietary selection year-round, which allows us to regard such selection as the staple FBF according to the definition proposed by Marshall and Wrangham (2007).

| Nutritional character and ecological adaptation
Lichens have a crucial nutritional value for R. bieti, considering their dominant high proportion in annual dietary composition, especially in the dry season, and being regarded as FBF with more than 80.0% of dietary composition (Table 1).Some components, such as fibers, usually contain the elements of NDF, mainly cellulose, and hemicellulose, which are indigestible in normal animals (Lewis et al., 2005).However, R. bieti can hydrolyze them into digestible volatile fatty acids by unique microbial fermentative processes in their multichambered stomach (Li et al., 2023;Xia et al., 2022).Therefore, lichen with high NDF would be a thriving food and nutrient resource for R. bieti.Tannin is always avoided in food selection in primates since it is difficult to absorb (Chivers & Langer, 1994;DeGabriel et al., 2009).That may explain why R.
bieti prefers Bryoria spp.over U. longissima, a selection of the low tannin level but higher NDF.
Usnea longissima contains significantly higher portions of fat, ADF, energy, and tannin than Bryoria spp.; the latter includes a considerably higher amount of DNF and ash (Table 3).However, after such numbers have been adjusted, referring to the actual amount consumed by R. bieti (SUM, including AB and AU), monkeys eat significantly higher proportions of Bryoria spp.rather than U. longissima, simply because the former occupies a much higher physical amount consumed (Table 3).
The study also reveals that R. bieti progressively reduces taking energy from TNC, NDF, starch, CP, ADF, WSC, ash, fat, and ADL, and the minimum amount of a chemical element (tannin, Figure 4).
During the dry season, the creatures significantly consume more of those components from lichens, which, as addressed above, is due to the scarcity of other food resources in the season.That also implies that R. bieti receives more components from non-FBF (fruits, seeds, and leaves) during the wet season (Figures 2 and 3).Thus, consuming lichens in the dry season, the only available food resource, is vital for R. bieti to cope with dual stresses: nutrition requirements and low temperature.Taking lichens, which have high availability and wide distribution in the dry season, is an evolutionary and ecological adaptation; avoiding monkeys consumes unnecessary energy loss due to traveling long distances to seek alternative foods.This energyconserving strategy is also very critical for other taxa in the genus in dealing with harsh ecology and unique periods; for instance, R.

| The implication of conservation
In this study, lichens play a vital food supply for survivability and future development for R. bieti and the other two species in the same genus (Figure 5)-a unique evolutionary development, ecological selection, and environmental adaptation.Among them, R. bieti faces a remarkable conservation challenge due to the significantly declining lichen quality, production, and fragmentation (Grueter, Li, Ren, Wei, & van Schaik, 2009;Grueter, Li, Ren, Wei, Xiang, & van Schaik, 2009).It has been reported that a renewal cycle of lichens, if damaged, could take as long as 21 years (Kirkpatrick, 1996;Seaward, 1987).The significant impacts on lichens include: 1.Over the decades, human-induced activities have prominently increased through deforestation, overgrazing, and cropland reclamation, which has caused soil erosion and rocky desertification.Such situations in the mountains where the species is located have become more severe since the 1990s, resulting in a sharp decline in biodiversity, frequent natural disasters, and severe ecological degradation.They have severely impacted lichens' quality and fragmentation (Jiang et al., 2003;Shang et al., 2018;Yin et al., 2020), and triggered significant natural resource exploitation, land conversion, and pasture extension, which have led R. bieti, like any other primate taxa, to face prominent conservation pressure in China (Pan et al., 2016;Xiao et al., 2003), especially suffering from the fragmentation of the lichens, soil erosion, and other ecosystems (Grueter, Li, Ren, Wei, Xiang, & van Schaik, 2009;Marshall et al., 2009).
2. The increasing air pollution has negatively impacted the growth and survivability of lichens in the regions (Cao, 1989).The Mts.
Hengduan, where the studied species dwells, are considered one of the essential regions of carbon sinks in South and Southwestern Eastern Asia.Thus, the ecosystems of many animals and plants are susceptible to air pollution caused by human-induced activities and natural modifications (Huang et al., 2020;Yin et al., 2020).
Lichen is very sensitive to the environment and ecology, especially 3. Climate change has placed another prominent pressure on lichen production.Evolutionarily, lichens appeared in about 600 mya (Yuan et al., 2005).However, they have changed significantly due to climate modifications from icehouse to greenhouse conditions, particularly during the Quaternary glacial and interglacial cycles (Ellis, 2019).Research indicates that from 1954 to 2009, the annual temperature in the region (Mts.Hengudan) increased by 0.9°C and the annual precipitation decreased by 185 mm (Chen et al., 2012).Consequently, lichens and many other non-vascular epiphyte species are heavily negatively impacted due to their very high sensitivity to temperature and moisture following climate change (Song et al., 2012).
Thus, the results of this study indicate that this and the other two species of snub-nosed monkeys depend essentially on lichens, as FBF are facing prominent challenges due to the damages and reduction of the lichens that are, however, most vulnerable to climate change, air pollution, and human activities.Increasing attention to the protection of lichens is critical to mitigate conservation pressures on the R. bieti.
The results found in this study can provide a scientific reference to maintaining balanced nutritional and chemical components for the monkeys, which have developed during their evolutionary and phylogenetic progress and are the consequence of ecological adaptation.Since this primate species has been categorized as a critically endangered primate species, captivity-feeding programs have been applied in some places (He & You, 2015).Alternative food resources have been used in the case of lichens being absent.Thus, it is critical to maintain their nutritional and chemical balances by referring to Figure 4 to select alternative dietary components and plant species for captivity breeding.

R
. bieti is singled out for its unique dietary selection-the highest lichen and the lowest leaf, fruit, and seed proportions compared with other Asian colobines.That can be regarded as a specific evolutionary development and the solution of environmental adaptation and ecological selection in the highest habitat elevation and harshest survival conditions of any other nonhuman primates on this globe

(
Mts, Hengduan) and plateaus (the Qinhai-Tibet and Loess linking the Qinling).Such scenarios must be relevant to the unique geographic and habitat choices after settling in East Asia under natural selection and environmental adaptation regulations and principles.Populations of R. strykeri in Northern Myanmar have a lower altitude (1720 m), and R. avunculus is primarily located in the forests of limestone mountains in northern Vietnam (Boonratana & Canh, 1998), which are quite F I G U R E 4 The nutritional and chemical components consumed by R. bieti, between dry and wet seasons, based on SUM values.F I G U R E 5 Dietary selection among Asian colobines.Database resources: Appendix Table to atmospheric nitrogen (N) deposition that has increased in recent years in China, which can significantly disturb the nutrient balance of the lichens besides constraining their growth and survival.The lichens' thallus growth and propagule survival can substantially decrease when nitrogen addition changes from 6.25 to 50.0 kg N•ha −1 year −1 .Further, lichen biomass could be reduced by 11.2%-70.2%when the deposition addition exceeded 6.25 kg N•ha −1 year −1(Yin et al., 2020).
. Thus, R. bieti, different from other primate species TA B L E 1 . Surviving at the highest and coldest: Nutritional and chemical components of fallback foods for Yunnan snub-nosed monkeys.Ecology and Evolution, 14, e11219.https://doi.org/10.1002/ece3.11219Food repertoire of R. bieti at Mt. Lasha.Data sources of dietary selection among Asian colobines.